Voltage references are necessary in almost all integrated circuits. One well known circuit configuration of a voltage reference is shown in FIG. 1. See Gray and Meyer, "Analog VLSI Circuit Analysis", Chapter 12, Wiley (1984). In this circuit, the two n-MOS transistors M1 and M2 have the same size (i.e. same W/L) and are biased by current sources 103 having the same magnitude. The gate of M1 is grounded. An operational amplifier (op-amp) is connected to the source sides (for detecting the difference of V.sub.t1 and V.sub.t2) and the op-amp output is connected to the gate of M2 for maintaining the M2 transistor at turn-on. The V.sub.t (threshold voltage) of the two transistors is made different by either channel implant or by different doping type of the poly gate. The output V.sub.o from the op-amp is simply the difference of the V.sub.t of the two transistors, i.e. V.sub.t1 -V.sub.t2. The accuracy of the circuit lies in the size matching of the transistors M1 and M2 and the offset of the op-amp. The basic circuit configuration in FIG. 1 can be modified with various additional circuits for trimming or calibration purposes, and the circuit is widely used in CMOS VLSI. The temperature coefficient of this circuit can be very good due to the cancellation of V.sub.t variations of the n-MOS transistors.
However, the circuit of FIG. 1 can provide only a single output voltage reference level that is set by the threshold voltages of the transistors. What is needed is a voltage reference circuit that is tunable over a wide range.